The instant invention is intended for use with precision motion systems such as coordinate measuring machines ("CMM"), large machine tools, semiconductor manufacturing equipment (e.g., mask alignment) and the like. Generally, these systems utilize a movable table having a precision ground and lapped working surface which slidably engages one or more stationary guideway surfaces for horizontal movement. While permitting longitudinal motion, vertical and lateral displacement of the table are substantially restricted. In many applications, these systems use servo-controlled drive systems or the like to permit precise linear positioning of the table.
To ensure accurate table movement, a bearing system is provided. In smaller systems, conventional linear bearing assemblies (e.g., roller bearings) provide adequate support without introducing excessive rolling resistance (friction). However, with larger motion systems having massive tables, conventional mechanical bearings are insufficient due to their large size and significant rolling resistance. To overcome these problems, air bearings are frequently used.
In the context of a motion system, an air bearing is generally a series of bearing "pads" which sit between the table and the guideways. Each pad has a backside which operatively couples to a slide portion of the table by fastening, vacuum coupling, adhesive or other acceptable means. The pad further has a face side which forms the bearing surface. The face side includes one or more openings or "pockets" oriented normal to the guideway surface. The pocket is generally coupled to a pressurized air source such that, when the air source is activated, pressurized air is delivered thereto. To create a relatively stiff bearing, it is advantageous to restrict the air flow through the pocket. This is typically accomplished with a restricting orifice located within the bearing pad. Once the pocket becomes pressurized, air escaping from the pocket to atmosphere creates an air film between the bearing face and the guideway surface. This air film permits the table to "float" and move relatively friction-free along the guideways.
While one group of bearings "lifts" the table, another bearing or group of bearings provides a downward force to oppose or "preload" the lift bearing. Alternatively, the lift bearing may be preloaded by utilizing spring-loaded means to couple the pad to the table slide. Counteracting side bearings are also provided to limit lateral table motion. By adjusting the pocket size, the number of pockets and pads and the air pressure, tables of most any size and weight can be adequately supported and guided. In addition, because air bearings are non-contacting, frictional forces are minimal. An example of a motion system that utilizes air bearings is shown in U.S. Pat. No. 4,234,175 issued to Sato et al. on Nov. 18, 1980.
One problem inherent with air bearings is the compressibility of the air medium. To produce a stiffer bearing, it is advantageous to minimize the bearing clearance (distance between the bearing pad and the guideway surface) as this reduces the volume of compressible air separating the components. However, decreasing the bearing clearance requires that the bearing pad, guideways, and table be machined and aligned to more exacting tolerances. Otherwise, the varying clearances between the moving components may result in unintended contact between the moving table and the guideway (i.e., "crashing"). Therefore, the air bearing system designer is often required to sacrifice bearing stiffness (increase bearing clearance) in order to maintain reasonable machining and assembly tolerances.
While conventional air bearings are more than adequate for many applications, problems remain. One problem in particular is attributable to the dissimilarity of the bearing material relative to the other system components. For example, the table and various other components are, in some systems, constructed of granite or diabase. These materials are preferred because they are thermally stable (i.e., they have a relatively low coefficient of thermal expansion or CTE) and they have excellent vibration damping characteristics. However, the bearing material itself is frequently metallic. As such, its CTE is much higher. The higher CTE results in the bearing pads expanding and contracting at a different rate than the granite. This can result in unintended restriction or expansion of the bearing clearance. If unaccounted for, this expansion can cause contact between the bearings and guideways and adversely affect the accuracy of the table position. Additionally, if localized temperature increases are experienced, the bearings may expand differentially, causing the table to shift and potentially crash into the guideways. Metallic bearing pads are furthermore subject to corrosion and thus may require periodic inspection and replacement.
Another problem inherent with conventional air bearings is the pocket itself. While the pocket provides a larger area over which air is distributed, it also increases the volume of compressible air supporting the table. Accordingly, the pocket limits the maximum bearing stiffness.
One method used to reduce or eliminate the dissimilar material problem discussed above is to eliminate the bearing pad altogether and incorporate the air bearing directly into the granite members. That is, passageways drilled in the granite couple the air source to bearing pockets formed in the granite itself. While the pocket diameter may be used to restrict the air flow, it is often of large diameter due to the drill size required to adequately form the pocket. Accordingly, the system may incorporate a plug having a small orifice thereon wherein the plug is inserted into each opening such that it is recessed from the bearing surface. Although such integral air bearings eliminate potential CTE mismatch and corrosion problems, the air column formed within the pocket still limits the maximum stiffness of the bearing.
Thus, there are unresolved issues with current motion system air bearings. What is needed is a bearing system for precision motion systems that permits minimal bearing clearance and improved bearing stiffness while preventing contact between the table slides and guideways. What is further needed is a bearing system that is capable of maintaining the desired bearing clearance regardless of temperature variations.